Stepper motor systems commonly employ two phase synchronous motors in combination with an incremental drive system. The normal stepping sequence is achieved by energizing phase A with a positive signal, then energizing phase B with a positive signal, then energizing phase A with a negative signal, then energizing phase B with a negative signal and then repeating the sequence. The rotor of the motor is advanced incrementally in a four step sequence. The motor itself preferably includes a large number of poles so that one revolution of the motor includes a large number of incremental steps. The motor is also designed to provide significant stationary torque so that the motor will maintain positions without overheating. An example of such a motor is disclosed to U.S. Pat. No. 4,330,727. Since the control of the motor in such a system is in an on/off, or generally digital fashion, harmonics in the motor torque/displacement characteristic have little effect on the system operation and are of little concern in the system design.
More sophisticated stepper systems, however, employ a control technique called "microstepping" where the motor can be controlled for positioning at virtually any desired position intermediate the normal incremental positions. This is achieved by proportioning the signals applied to the phase A and phase B windings to obtain a field vector as required for the intermediate points. In such a system, since the control is basically analog rather than digital, harmonics have a substantial effect on control accuracy and are of great concern.
Harmonic suppression in motors has been known in the past. Typically, two phase motor designs inherently suppress the second harmonic whereas three phase designs suppress the second and fourth harmonic. Those even harmonics exist without current in permanent magnet motors.
In motors designed for stepper operations, considerable work has been carried out attempting to eliminate harmonics through careful control of the tooth shape of the magnetic pole pieces. This approach does achieve harmonic reduction but requires very exacting machine tolerances and difficult shapes and, hence, is not practical in most cases. Also, the tooth shape that suppresses one harmonic often tends to enhance other harmonics and therefore suppression of all significant harmonics is difficult or impossible to achieve in a practical design.
In the past, techniques have also been employed for reducing the detent or cogging forces in motors by skewing the motor slots. Although not generally regarded as such, this technique can be viewed as a form of harmonic suppression, since the cogging or detent forces result from field distortions in the motor. Skewing angles have usually been arrived at in a cut and try approach to minimize cogging without any conscientious attempt at cancelling or suppressing specific harmonics.
Reduction of static detent forces in single phase step motors, through pole position shifting to suppress the fourth harmonic is also known. See for example U.S. Pat. No. 4,155,018. Such single phase motors without the shifted poles cannot produce effective starting torque from certain rest positions. The shift in the pole positions to reduce the fourth harmonic tends to shift the static equilibrium mode point to a position where a larger starting torque is generated.
Microstepping systems have also been designed with "compensated" proportioning of the energizing signals. According to this technique, the deviation from the ideal position is measured for each microstep position and the proportioning of the energizing signals then adjusted accordingly. Unfortunately, with this technique the compensation is correct only for one set of torque and speed values and is not effective where the system must operate over a broad range of conditions.